Pink lines aren't getting any darker?

So I got my big FAINT positive at 11 DPO. 😄 I've been testing every day since, but it doesn't look like the lines have been getting any darker for the past 3 days. Is this normal? I read that HCG levels are supposed to double every 48 hours or something like that. Shouldn't the line be getting darker? I'm hoping to see a line as dark as the control line, but maybe that doesn't come until a little while after my missed period? Any input would be greatly appreciated! 💜